Cut a leather gusset, cut a slot in it for the zipper and stitch the zipper into it. Cut the gusset a couple inches longer than the edge you are connecting it to. Run the ends of the gusset down into the spine, as if it were a book cover. Find the exact centre of the gusset, and exact centre of the sides of the purse and glue or temporarily stitch or staple in place. You can have the zipper unzipped while doing this. Line it all up, glue or staple in place with the gusset wrapped down inside. Don't stitch the ends of the gusset into the spine, but let it float. Sew it up, done.
Tom
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If you want to keep the purse real thin, just use the zipper like you did, but get one long enough to go about an inch around the two end corners, and cut a slot for the zipper to fit into. I would still find the exact centres to match up so the zipper is not accidentally distorted and hard to zip.